1. Field of the Invention
The present invention relates to a photoreceptor for electrophotography that contains amorphous silicon.
2. Prior Art
The life of a photoreceptor for use in electrophotography is known to be chiefly governed by such factors as the deterioration of its electrical properties, the occurrence of flaws on its surface, and the changes (especially the thermal change) in the properties of the materials of which the photoreceptor is made. Photoreceptors made of amorphous silicon based materials have recently been the subject of intensive studies by many researchers because it is anticipated that such materials will be completely free from the restraints of the various factors that have governed the life of conventional photoreceptors. In other words, since amorphous silicon materials retain stable electrical characteristic over cyclic use, have high hardness, and are thermally stable, they have the potential to provide an extremely long-lived photoreceptor.
Beside its potential for extending the life of photoreceptors, amorphous silicon has a high photosensitivity in the range of longer wavelength than conventional materials and its sensitivity can be further extended into the range of still longer wavelength by selecting an appropriate formulation. Therefore, photoreceptors made of amorphous silicon can be used with printers that employ small and low-cost semiconductor lasers as light sources.
In spite of these advantages that increase its potential for use as the material of a photoreceptor, amorphous silicon has its own problems in practice in terms of dark resistance, photosensitivity at long wavelengths, mechanical strength properties (in particular, ductility), time-dependent stability, and dependency of image quality on environmental factors (i.e., temperature and humidity).
Amorphous silicon materials have high hardness (their Vickers hardness is on the order of 10.sup.3) but if they are brought into contact with less hard materials (e.g. the edge of copying paper and the cleaning blade in a copying machine), the area of contact will fail to produce an image and remain as white dots. It is also known that a photoreceptor made of amorphous silicon experiences a reduced resolution (i.e., dilation) if it is cyclically used for fairly long period in a copying machine (or printer). This is probably due to the deposition of foreign matter on the surface of the photoreceptor and/or to the change in the proterties of the photoreceptor. The phenomenon of dilation can also materialize for reasons associated with the structure of the photoreceptor (e.g. use of an inappropriate surface layer) and if this is the case, the phenomenon will occur in the initial period of use, that is, within a few cycles to several tens of cycles of operation.
The applicants of the present invention previously resolved the aforementioned problems by proposing an amorphous silicon photoreceptor having two amorphous silicon surface layers containing different concentrations of nitrogen atoms as disclosed in U.S. Ser. No. 061,964 filed on June 15, 1987.
However, the above photoreceptor, if the overall film thickness of the surface layers is set so as to satisfy the resistance to printing which is required according to the various conditions in a copying machine or a printer used, it has been difficult to satisfy the requirements for the residual potential and the sensitivity to a short wavelength light (in the vicinity of 500 nm). That is, the residual potential is proportional to the concentration of nitrogen atoms in the surface layers, while the absorption coefficient of the surface layer for the short wavelength light is inversely proportional to the concentration of nitrogen atoms. Accordingly, when the concentration of nitrogen atoms is lowered to reduce the residual potential, there occurs a problem that the sensitivity to light is reduced due to absorption by the surface layers.